One class of touch sensitive screens or sensors of the prior art utilizes two parallel conductive sheets which are held a short distance apart so that they do not normally make electrical contact. In operation a voltage is applied to one of the sheets so as to produce a "gradient" sheet, allowing current to flow therein. When the other sheet, referred to as a "sensing" sheet, is touched, it makes contact with the gradient sheet and so can be used to sample and measure the voltage on the gradient sheet at the point of contact. While these sheets can have different orientation, the gradient sheet is usually the bottom of the two sheets. The voltage thus produced is an analog of the position of the touch on the screen, and thus the position of the touch can be sensed. Typically a voltage is impressed in one direction across the gradient sheet during one short time interval with the touch voltage being measured. The voltage is then impressed in an orthogonal direction across the gradient sheet for another short time interval with the touch voltage again being measured. These two measurements completely define the position at which the screen has been touched. This alternation of direction of application of voltage to the gradient sheet occurs typically about 100 times per second thereby providing a substantially continuous stream of position information.
As is known by those skilled in the art, the required rapidly alternating voltage can be applied to the gradient sheet in several different ways. Most directly, a system of diodes can be placed at intervals along the edges of the gradient sheet and the voltage fed to the sheet through the diodes. These diodes also serve to block the flow of current in the wrong direction during alternating cycles.
As an alternate means, a resistor chain can be placed along the sides of the sheet with leads at intervals run onto the sheet to serve as feeder lines. Such a chain must serve a dual function. When a voltage is to be impressed in one direction, e.g., top to bottom in the sheet, the resistor chain across the top serves as a voltage feeder, with the same reference voltage applied at both ends of the chain and, similarly, the chain across the bottom is grounded at both ends. When this is occurring, the resistor chains down the sides of the sheet have the reference voltage at the top end and the ground voltage at the bottom; thus, the leads coming in from these side chains must be calibrated to be at the appropriate voltage for that position. These resistor chains, too, have been produced in several ways. In one, a resistive wire has been placed along each edge of the sheet as described in U.S. Pat. No. 4,661,655 issued to W. A. Gibson, et al on Apr. 28, 1987. With this construction, by running leads directly to the sheet at equal spacings along the wire, the needed voltages are produced. Other resistor chains, which function in a similar manner, are described in U.S. Pat. No. 4,731,508 issued to W. A. Gibson, et al on Mar. 15, 1988; and U.S. Pat. No. 4,822,957 issued to J. E. Talmage, Jr., et al on Apr. 18, 1989. The latter type of resistor chains are formed using an overlap conductor array on the conductive gradient sheet itself (the gradient sheet having some resistivity).
Because the resistor chains alternate functions when the voltages on the gradient sheet are switched (from top-to-bottom to across-the-sheet), it follows that the same resistor chains must also serve to feed the voltage to the sheet at the top and drain it at the bottom. Unaltered, this configuration does not produce an acceptable touch sensor because the current must flow through more of the resistor chain to get to the center of the sheet than it does to reach the corners; thus, the voltage at the center of the sheet will be lower than at the edge. This effect is called "bow" and, if the output voltage from the sensing sheet is to be used as a direct analog for distance (position), the bow effect must be eliminated or at least minimized. As taught in the above-cited U.S. Patents, all of which are assigned to the assignee of the present touch sensor, one method for substantially overcoming the bow is to vary the size of the contacts (electrodes) being fed via the leads from the resistor chain to the sheet. Typically, these contacts are "T" shaped (called tees hereinafter), with the top cross portion of the tee feeding voltage to the sheet. The wider the tee with respect to the width of the portion of the sheet to which it is feeding voltage, the lower the apparent resistance to current flow. Thus, by placing wider tees at the center of the sheet and narrower tees toward the corners, the total series resistance which the reference voltage encounters before reaching the active area of the sheet can be adjusted so that the bow is eliminated.
Unfortunately, it has been found that this method of bow elimination produces two adverse side effects. First, as the tees are necessarily made more narrow toward the corners of the sheet, a phenomenon called "ripple" is produced. The voltage contours tend to cup around the voltage source tee, and the cup is much more pronounced when there is a large open space between tees. A second variety of ripple is caused because, while the tees should match specific reference voltages along the side of the sheet, only the center of the tee can be at the correct reference voltage. Thus, constant voltage lines running across the sheet which are slightly above or slightly below the voltage of the tee must suddenly deviate so that ripple is also produced by tees which are too wide.
The second important side effect of the resistance chain and tee design comes from the consequence that a resistance chain feed element cannot be placed at the corner of the sheet active area. This means that the corner of the sheet, farther toward the corner than the last tee, is a "bad" area. This often requires that the first tee of the string must be placed very near the corner and must thus be quite narrow, which increases the undesirable ripple in the corner. Accordingly, the touch screens designed according to the above-described prior art lack accuracy in the corner areas.
Accordingly, it is an object of the present invention to provide a touch sensitive screen in which the bow of equipotential lines therein has been substantially eliminated and in which the ripple of the equipotential lines in corner areas of the screen have been substantially reduced so as to improve accuracy of response in these areas.
It is another object of the present invention to provide a touch sensitive screen having resistor chains along each edge, with leads to contacts on the surface of the screen connected to these resistor chains at selected points to produce selected voltages on the screen during operation thereof, with the contacts on the surface having selected spacing and effective length to compensate for any cumulative voltage drop along the resistor chains so as to substantially eliminate bow in equipotential lines in the screen, the screen further having a contact of a selected length in each corner, the corner contact not being a part of the resistor chain but a part which operates electrically in parallel with the resistor chains.
These and other objects of the present invention will become more apparent upon a consideration of the drawings referred to hereinafter when taken together with a full disclosure of the invention.